1. Field of the Invention
The present invention relates to a cutting tool holder, especially to a cutting tool holder for a machining center.
2. Description of the Prior Arts
A machining center is a kind of machine that can perform cutting, drilling, boring, scraping steps etc. With reference to FIG. 7, a machining center comprises a circular tool rack (50). The circular tool rack (50) has multiple first conventional cutting tool holders (51). The cutting tool holders (51) are mounted around the circular tool rack (50). When the circular tool rack (50) is rotated, the location of the cutting tool holders (51) is accordingly changed. With further reference to FIG. 8, each cutting tool holder (51) has a conical bore (511), a proximal end, a wall, a through hole (510), four screw holes (512) and four fastening devices (55). The through hole (510) is in communication with the conical bore (511) and extends to the proximal end of the cutting tool holder (51). A diameter of the conical bore at a distal end of the tool holder (51) is larger than a diameter of the through hole (510) in the proximal end. The screw holes (512) are formed through the wall, are near the proximal end of the cutting holder (51) and perpendicularly correspond to the through hole (510). The fastening devices (55) are respectively mounted in the screw holes (512) and each has a ball bearing (551), a spring (552) and a screw bolt (553). The ball bearing (551) is mounted in the screw hole (512) and protrudes to the through hole (511). The spring (552) is mounted in the screw hole (512) and presses against the ball bearing (551). The screw bolt (553) is mounted in the screw hole (512) to keep the spring (552) and the ball bearing (551) from escaping the screw hole (512). A cutting tool (61) is mounted on a seat (60). The seat (60) has a proximal end, a distal end and an annular recess (601). A diameter of the proximal end of the seat (60) is larger than a diameter of the distal end of the seat (60). The annular recess (601) is formed around the seat (60) near the distal end. The seat (60) is inserted in the through hole (511) and the annular recess (601) receives the ball bearings (551). Because the springs (552) press against the ball bearings (551), the fastening device (55) can securely retain the seat (60).
With further reference to FIG. 9, a different kind of cutting tool holder (51′) is needed to correspond to the different kind of seat (60′). If the annular recess (601) is formed on the seat (60′) at a different location, the location of the screw holes (512) on the holder (51′) has to be changed. However, drilling the screw holes (512) through the wall and forming the thread are difficult operations.
With reference to FIGS. 10 and 11, a second conventional cutting tool holder (70) comprises a through hole (71) and a fastening device (80). The through hole (71) is formed through the cutting tool holder (70). The fastening device (80) is mounted in the through hole (71) and has a central hole, four conical holes (81), four ball bearings (82) and four springs (83). The conical holes (81) are formed through the fastening device (80) and correspond to the central hole. The ball bearings (82) are respectively mounted in the conical holes (81). The springs (83) are respectively mounted in the conical holes (81) and press against the ball bearings (82). Although the fastening device (80) of the second conventional cutting tool holder (70) does not require the thread, the second conventional cutting holder (70) still has numerous shortcomings. Because the springs (83) easily escape from the holes (81), it is difficult for the fastening device (80) to be mounted in and dismounted from the through hole (71) of the cutting tool holder (70). The ball bearings (82) and the springs (83) are easily lost when changing the fastening device (80).
To overcome the shortcomings, the present invention provides a cutting tool holder for a machining center to mitigate or obviate the aforementioned problems.